The present invention relates in general to identifying signal quality of a received radio transmission, and more specifically to an indicating circuit for detecting the presence of a strong FM signal without the presence of noise or interference. The present application is related to U.S. Pat. No. 5,073,976 filed concurrently herewith.
The measurement of received signal quality has been employed in FM receivers for implementing a number of different functions. For example, automatic scan tuners which sweep through the FM broadcast band must terminate a scan when a broadcast signal is received having a particular signal quality. Signal quality is also measured in order to modify receiver characteristics in response to certain conditions, e.g., output blanking or reducing stereo separation in response to the presence of noise.
Several different methods have been employed to generate a stop signal for terminating the sweep operation in an automatically scanning tuner. Most methods rely on a measure of the received signal strength, or a frequency window determined by the automatic frequency control (AFC) signal, or a combination of both. Integrated circuits are commercially available which provide FM intermediate frequency (IF) demodulating systems These systems include signal strength level detectors and AFC window detectors that are or can be interconnected to generate stop pulses for scan tuning operations (e.g., the CA3209E integrated circuit made by RCA Solid State Division and the TDA 4220 integrated circuit made by Siemens).
The presence of a received signal strength greater than a predetermined magnitude provides an insufficient indicator of signal quality to stop a scan tuning operation because the received signal strength may be high while signal quality is low due to the presence of noise, adjacent channel interference, or multipath interference. Even with the use of a frequency window to ensure that scanning is stopped accurately at the frequency of a strong received signal, stopping on a received signal which includes noise and interference continues to be a problem.
It is also known to examine the noise component of a signal relative to the information signal component in order to determine signal quality. For example, communications transceivers employ output inhibiting or squelch when a particular level of noise is present. However, the prior art systems measure noise at frequencies that also contain information-related components. Therefore, only noise that is substantially equal to or greater than the information component can be detected.